What's so funny about the TV networks' new fall schedules? Everything — or at least that's the hope of programmers who unveiled their new lineups to advertisers this week in New York and have gone cuckoo for comedy in a way not seen in at least 15 years.

Reversing their lament from a few seasons ago that sitcoms were suffering from a creative and ratings drought, the networks are now whipping up a cloudburst of hoped-for laughs that will rain across flat screens, laptops and tablets come September.

Desperately seeking hits, NBC alone will have 10 comedies this fall, four of them new.

In a funny-bone pileup during the 9 p.m. Tuesday block, three of the Big Four broadcasters will be airing sitcoms, with No. 1-ranked CBS the lone exception. During a breakfast session with reporters, CBS scheduling chief Kelly Kahl predicted that rivals were headed for a "comedy Sig-Alert."

"Everyone seems to believe that sitcoms are the bedrock of big-time success for the networks," said Bill Carroll, vice president at New York-based Katz Media Group, which helps advise local TV stations, earlier this week.

What's behind the joke rush? Partly it's the success of CBS' "The Big Bang Theory" and ABC's "Modern Family," two smash hits that have proved the appetite for sitcoms — or at least well-made ones — still exists.

Of course, this isn't the first time network executives have tried to laugh their troubles away. Back in the 1990s, when "Friends" became a surprise hit among young affluent adults, NBC and its rivals spent tens of millions of dollars trying to make lightning strike twice.

By the fall of 1997, NBC had an astonishing 18 comedies on its schedule and ABC and CBS had 12 apiece, according to ad firm Horizon Media.

That statistic makes this coming season's laugh track seem subtle in comparison. But at a time when broadcasters are losing their grip on young viewers — the ratings this spring have been especially tough — the thinking is that comedies can help lure them back.

Fox's freshman sitcom "New Girl" with Zooey Deschanel has a median viewer age of 35 — at least 10 years younger than the typical prime-time show, according to Nielsen.

An increase in the number of young viewers means a hike in the prices networks can charge advertisers, which are willing to pay more for young adult audiences than for middle-aged or senior ones.

Also, viewers tend to show up more often for repeats of sitcoms than they do for encore reality shows and serialized dramas. That means networks can squeeze more value out of the tens of millions of dollars in license fees paid to studios.

"Half-hour comedy shows are easier and cheaper to gear up than dramas, so there is less overall investment and risk than (in) developing new dramas," said Jeffrey McCall, a communications professor at DePauw University.

But everyone realizes the networks' best weapon will be what it's always been: a new hit show. Whether the comedy-centric fall schedules will deliver on that score, however, remains to be seen.

With its strong ratings, CBS has the luxury of making the fewest changes to its lineup, with just four new series, including the Sherlock Holmes update "Elementary" — with the detective this time played by Jonny Lee Miller and Watson played by a woman, Lucy Liu.

But maybe the bolder move is moving the landmark sitcom "Two and a Half Men" from Mondays — where it premiered in 2003 and became an anchor for the night — to Thursdays. The new CBS entry on Mondays will be "Partners," a buddy comedy from the creators of "Will & Grace." As the lead-in to "2 Broke Girls" — this season's top-rated new show — "Partners" will get crucial exposure.

Fox, the leader among young-adult viewers, is bringing back its two high-rated singing contests, "The X Factor" and "American Idol."

Both have reputations for unpredictability (which might be good for the show) and career-interrupting personal problems (which might not).

Meanwhile, "Idol's" ratings have swooned nearly 30 percent this season and the producers have yet to reach a deal with judge Jennifer Lopez, who has made noises about leaving.

Fox has also jumped aboard the comedy train, with new Tuesday entries "Ben & Kate" and "The Mindy Project."

ABC is holding a mixed bag. Its standbys will return, namely "Dancing With the Stars" and "Modern Family." The fantasy "Once Upon a Time" has shown promise this season and, with "Desperate Housewives" gone, it will hold down Sundays in the fall along with the soap "Revenge" and a new supernatural thriller, "666 Park Avenue."

But the network failed to find the clear breakout hit during the 2011-12 season that could have helped it avoid a battle with NBC for last place.

For fall, ABC has ordered just one comedy, although it's a high-risk one: "The Neighbors," an unusual offering about a normal American family that moves into a gated community populated by space aliens. In a sign of network confidence in its prospects, it will air in the key slot just after "Modern Family" on Wednesdays.

Finally, there's NBC, which will enter the fall with two key assets, NFL football and its singing show "The Voice."

Unfortunately, the Sunday gridiron games disappear in January, while "The Voice" saw its ratings sink this spring and it's anyone's guess how it will fare during a crowded fall rollout (its first two seasons were during the summer and the spring).

NBC ordered four new comedies. "Go On," with ex-"Friends" star Matthew Perry as a sportscaster, will be paired Tuesdays with "Glee" executive producer Ryan Murphy's "The New Normal," a sitcom about a gay couple.

On Wednesdays, "Animal Practice," a comedy about a young veterinarian, will lead into "Guys With Kids," about thirtysomething dudes struggling to be family men.

However, putting new comedies back-to-back is considered highly risky by TV schedulers, who have long believed it's easier to sell a half-hour series if viewers already recognize the one that comes before it.

If fall produces one or more new smash sitcoms, viewers might, under the inexorable logic of the TV industry, have to brace for even more comedy.

And if all these chuckle fests fail? Television will roll on in search of its next source of programming gold.